#521: Enrich synthetic data set with shared blocks (#522)

* #521: description, constructing script, and dataset files

* #521: Configuration file to run the synthetic+blocks case through PhaseStepper and render it

* #521: modified example and specification

* #521: new test case

* #521: excluse pngs from trailing whitespace check

* #521: fix png whitespace error

* #521: pull in LBDatafile_schema.py with JSON_data_files_validator.py

* #521: finalized example and configuration with baseline load-only setting that must reach 0 imbalance

* #521: generalize whitespace check to ignore all pngs

* #521: remove unused imports

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Co-authored-by: Caleb Schilly <[email protected]>